The present invention relates to gas sensor circuits employing semiconductors as the sensor element. It is known that the electrical resistance of certain semiconductor materials, such as tin dioxide for example, vary in accordance with the concentration of certain gas atmospheres, e.g., carbon monoxide, hydrogen, methane, etc. into which said semiconductors are placed and that it is therefore possible to utilize this transducer property of the semiconductor in gas sensor circuits. Examples of such prior art gas sensor circuits are found in U.S. Pat. Nos. 4,112,356 and 3,997,837. These circuits are characterized by the fact that the voltage output of the sensor is directly connected to the positive input terminal of a conventional differential amplifier (CDA). Unfortunately, a sensor's output voltage is often non-linear over a wide range of gas concentrations and hence the output signal of the CDA is also non-linear. Furthermore, this non-linear output often varies according to curves which are not easily defined mathematically.
This non-linearity has heretofore presented the prior art with a number of problems, expenses and work inconveniences. For example, since the output of the prior art circuit is non-linear, any output measuring device such as a voltmeter must be specially made or specially calibrated in order to accurately relate a gas concentration to a given voltage. On the other hand, if a linear output were being produced by such a gas sensor circuit then standard voltmeters could be used to measure the linear gas concentration to voltage relationship without need for the expensive calibration techniques required to measure a non-linear relationship.
Problems associated with the use of non-linear output signals are particularly evident in systems applications where such signals are to be fed to a computer. Because both analog and digital computers can directly use a linear input without the need for storing the calibration information associated with a non-linear relationship, great inconvenience and expenses can be avoided if the computer can be fed a linear signal. Therefore, rather than trying to follow such curves electronically most prior art gas sensor circuits were designed to operate above a certain discrete voltage point and to cease to operate below that same voltage point. Such single point activations severely limit the system's versatility because these single point circuits will not operate at gas concentrations lower than that necessary to lower the semiconductor's output voltage to the circuit's activation point. On the other hand, setting the voltage activation point to higher values may result in unnecessary operation of equipment such as parking garage exhaust fans, which are subsequently connected to the gas sensor circuit's voltage output. Heretofore, the solution to this dilemma has been to either be satisfied with one arbitrary operation point or to construct the sensor circuit with a series of different discrete voltage activation points or, in the systems situation, to store the non-linear curve information in the computer. Naturally, such solutions to the problem are more complex, and hence more costly.
U.S. Pat. No. 4,185,491 deals with the problem of non-linear output of such gas sensor units by connecting a known bridge type linearization circuit to its comparator amplifier and to its two differential amplifiers in order to achieve its purpose of avoiding unnecessary operation of the sensor at reduced temperatures in order to prevent sensor poisoning which occurs by the more rapid absorption of gas at lower temperatures. Known bridge linearization circuits of the type employed in this patent for the purpose of reducing operation of the sensor at reduced temperatures are also relatively complex and costly.